1) Ahrefs to quickly see “the big picture” when it comes to any keyword I'm researching. I can instantly see the top holders in the SERPs. I then immediately take the top holders list and go check out their sites. I need to make sure I can beat them content-wise, otherwise I will search for another keyword to try and rank for, or perhaps go down the long-tail route. The Ahrefs tool and data quality get better and better every year. It's one of my favorite tools.
It's not always about getting visitors to your site, but about getting the right kind of visitors. The usefulness of this intelligence cannot be overstated; with keyword research you can predict shifts in demand, respond to changing market conditions, and produce the products, services, and content that web searchers are actively seeking. In the history of marketing, there has never been such a low barrier to entry in understanding the motivations of consumers in virtually any niche.
This is a rather crude metric because it presumes one can monetize all the traffic they receive AND one can generate as much profit per visitor as Google does. Anyone who could do both of those would likely displace Google as the first consumer destination in their market (like how many people in the United States start ecommerce searches on Amazon.com rather than Google.com).

1 – I’ve found a niche with low competition and good CPC . but the thing is the while doing competition analysis. I observed that most of the sites that are in the top 10 serps ranking are small ecommerce site selling their own product or sites like niche specific classified ad site with low and thin content. What are your suggestions ? Should I go for such keywords ?

I learned a lot from you and now make a couple hundreds per month in extra cash thanks to your advice. You provided us with great content and a great tool. I’m very happy for you. I have to say though that I’m sad you sold… customer service is so so so far and I know it’s cool for owners to sell subscriptions instead of selling it for a one time fee and I know it’s a new trend online but it’s really a trend I don’t like as a customer… Anyways. I’m very happy for you. Many thanks and congrats !!!
3) KWFinder is one of the "newer" kids on the block, but it's probably just about the easiest way I have found to find new long-tail keywords quickly. A couple of things I like about this tool is that it allows me to create lists of keywords. So I can group up my different sites by lists and revisit them at a later date. I can export the data to CSV and start building out campaigns. It also keeps a nice scrolling list of the last 20+ keywords you have looked up. The SEO difficulty indicator comes in very handy as well! As far as ease of use goes, KWFinder wins hands down.

I also hang out in SEO forums and groups and see KC scores tossed around and talked about and pretty much everybody is on the same page. Most agree that you can’t completely rely on the KC score (there are a few factors that can’t quantitatively be measured with a number) – but that the KC score is a VERY good indicator of the competition level for specific keywords. The following diagram realistically shows exactly what to expect from certain KC scores…

It would be great if you can provide comparative analysis of this tool with Google Keyword Planner. Since you’re using this tool so you can provide best comparison. One differentiation which I understood is that this tool works for yahoo and bing as well whereas Google keyword planner works only for Google but Google being a dominating search engine, gives the highest returns, so only this reason may not be enough to buy it.